On Monday, Aug. 21, a total solar eclipse will be visible from a narrow strip of land that stretches all the way across the U.S., from Oregon to South Carolina. The event is expected to draw millions of people to prime viewing locations along its path. (Some 12 million people live within the path, but as many as seven million more may drop in just for the eclipse.)

If you’re even thinking of viewing the eclipse—a rare celestial treat often described as Nature’s greatest spectacle—you’re probably looking for answers to a few questions. And so here is NBC News MACH’s “Eclipse 101”—everything you need to know to have safe and enjoyable eclipse experience.

In a word: proximity. There hasn’t been a total eclipse visible from the contiguous 48 states since Feb. 26, 1979, almost four decades ago. Some people, like meteorologist Joe Rao, have had it marked on their calendars for years. “There are two generations of Americans who may have heard of eclipses—maybe they know what a total solar eclipse is, and what it’s all about—but they’ve never had a chance to witness it,” Rao says. “And now, finally, we have one that’s going to be visible coast to coast.”

A solar eclipse occurs when the moon passes between Earth and the sun, blocking the sun’s light and forming a fast-moving shadow on the surface of our planet. Because of the geometry of eclipses, the moon’s shadow is narrow when it reaches Earth, so the region from which the eclipse is visible is small. The shadow can never be more than 166 miles wide. During August’s eclipse, it will vary from about 60 to 70 miles wide.

As the moon moves in its orbit around Earth, this patch of darkness sweeps across Earth’s surface, and only people within this “path of totality” get to witness the total eclipse. (Lunar eclipses, in contrast, can be seen from anywhere on the side of Earth that’s facing the moon, as the moon passes through Earth’s shadow. So while lunar eclipses are just as rare as solar eclipses—they happen roughly once every 18 months—they’re typically seen by many more people.) People outside the path of totality can see a partial eclipse, with the moon covering only a portion of the sun’s disk.

A total solar eclipse in Belitung, Indonesia on March 9, 2016.AP file

There’s also a third kind of solar eclipse. Sometimes, because of the moon's orbital geometry, the moon isn’t large enough in the sky to block out the sun. A small ring, or annulus, of sunlight remains—causing a so-called “annular” eclipse. While partial eclipses and annular eclipses are interesting to see, it is only during the brief moments of a total eclipse that the sun disappears from the daytime sky, turning midday to night.

A solar eclipse begins slowly, with the moon’s disk gradually making its way across the face of the sun. These partial phases of the eclipse last about 90 minutes, and at first, it’s hard to discern that anything unusual is happening. Only when the sun is almost fully covered by the moon is it obvious that something strange is underway. The temperature drops. The landscape darkens. Shadows become sharper and colors more muted. Birds, confused by the midday darkness, start to squawk. Then the final bit of the sun disappears from view, and totality begins.

The moon now hangs in the sky like a black disk where the sun used to be. With the sun hidden, the sun’s outer atmosphere (solar corona) comes into view. The sky is a deep twilight—dark enough that Venus becomes visible, and other bright stars and planets may pop into view. If you’re on a hillside or mountain, you might be able to see parts of Earth that aren’t in the moon’s shadow, where the sky is lighter. The effect is sometimes described as a “360-degree sunset.”

Totality is brief, but exactly how brief depends on your location. On the Pacific coast, it’ll last one minute and 59 seconds. On the Atlantic coast, the figure is two minutes and 34 seconds. In between, the length of totality varies, reaching a maximum at a spot about six miles southeast of Carbondale, Illinois, where viewers can witness two minutes and 42 seconds of totality.

The closer you are to the “centerline” of the path, the better. As one moves from the centerline to the edge of the path, the duration of totality drops sharply.

NASA has produced detailed state-by-state maps of the path of the eclipse. The moment at which totality begins varies across the eclipse path (and not just because of the differing time zones). It occurs at 10:17 a.m. PDT in Salem, Oregon; at 11:33 a.m. MDT in Idaho Falls, Idaho; at 11:43 a.m. in MDT Casper, Wyoming; at 1:12 p.m. CDT in Columbia, Missouri; at 1:27 p.m. CDT in Nashville, Tennessee; and at 2:46 p.m. EDT in Charleston, South Carolina.

In terms of duration, Carbondale, Illinois is the big winner, as noted above. But even if you’re a couple of hundred miles to the east or west, you’ll lose no more than a few seconds of totality (as long as you’re near the centerline of the eclipse path). A more important factor is the weather. The best locations tend to be west of the Mississippi River. Precipitation and cloud cover data suggest that viewers in central Oregon, Idaho, and Wyoming have a roughly 60 percent to 80 percent chance of clear skies on Aug. 21. Your odds are only half as good on the eastern part of the path, from Missouri to the Atlantic coast.

But those are just averages. Check the forecast a few days ahead of time—and again, on the evening of the 20th—and be prepared to adjust your plans accordingly.

You probably won’t be going anywhere without a car, and your car will need a highway—which is why central Nebraska holds a particular appeal for eclipse watchers. That’s where one finds a strategically useful 250-mile stretch of Interstate 80, which runs roughly parallel to the path of totality. If one took up a position near Grand Island, for example, one could make a last-minute decision to drive 125 miles west to North Platte, or about the same distance east to Lincoln, in search of clearer skies, if needed.

For many people, the eclipse will be part of their summer holiday plans. So destinations such as Grand Teton National Park, in Wyoming, are likely to be a top draw. The park, which lies within the path of totality, features some of the country’s most spectacular mountain scenery—Yellowstone is right next door—and prospects for clear skies are quite good.

Grand Teton National Park may be a good place to view the eclipseGrand Teton National Park / NPS/Tobiason

Many hotels along the path of totality are already full. But all hope is not lost. A quick on-line search shows that (as of June 27) rooms are still available in Portland, Oregon (less than a two-hour drive from the path of totality); Salt Lake City (a four-hour drive); Denver (roughly a three-and-a-half-hour drive); and St. Louis, a city whose metropolitan area straddles the eclipse path.

Michael Zeiler, who runs the greatamericaneclipse.com website, estimates that between 1.85 and 7.4 million people will visit the path of totality on eclipse day. That’s in addition to the 12.2 million who live within the path. It would be nice if everyone spread out along the eclipse path, but it’s likely that they’ll end up clustered in locations where north-south interstate highways intersect the path—and thus certain “choke points” are inevitable, he says. Zeiler also cautions that his estimates may be on the low side—especially if local TV news and social media pump up the event in the days leading up to August 21.

Finally, essentials like gasoline and food could be in short supply within the path of totality. Zeiler’s advice: “Be as self-sufficient as possible. Bring food, bring water, keep the gas tank full, bring toilet paper, bring hats and sunscreen… it’s pretty much a given that the local facilities are going to be overwhelmed.”

The eclipse can be viewed safely, but you’ll need to take special precautions. Many people think there’s something inherently dangerous about the sun’s rays during an eclipse. In fact, the sun is no brighter during an eclipse than at other times—it’s just that people are more tempted to look.

Looking at the sun without proper protection can damage your eyes quickly, “with about a 50-50 chance of causing a loss of vision,” says B. Ralph Chou, professor emeritus of optometry and vision science at the University of Waterloo in Ontario. To look at the sun safely, he says, you need a filter capable of reducing the amount of light reaching your eye by a factor of 250,000. Inexpensive eclipse viewers that meet this standard are readily available. Start by asking for a pair at your local astronomy club, science museum, or planetarium. And thanks to the Space Science Institute, two million eclipse viewers will be available for free at public libraries across the U.S. They can also be purchased from many online retailers.

Don’t try to use sunglasses, smoked glass, or some other home-made substitute. They’re not safe.

If you’ll be viewing the eclipse through a telescope, binoculars, or telephoto lens, it’s essential to have a proper solar filter. It should be securely attached to the instrument’s objective lens (never just over the eyepiece).

Another tried-and-true option is to use a pinhole system to project an image of the sun on white cardboard.

Here’s the good news: during totality—the moments when the sun is completely blocked by the moon—the eclipse is perfectly safe to look at. As Chou writes in the American Astronomical Society’s eclipse safety guide, “the total phase of a solar eclipse can, and should, be observed without any filters, and certainly never by projection! It is completely safe to do so." (A good place to read up on eclipse safety is the AAS’s web page on the subject.)

Michael Bakich of Astronomy magazine urges people not to take pictures during the main event. “No picture will capture what your eyes will reveal,” he writes on the magazine’s website. Plus, fiddling with camera buttons is not the best use of your time during the fleeting moments of totality—especially if this is your first eclipse. “Watch your first eclipse with your mouth agape,” he writes, “where your only distraction is occasionally wiping tears of joy from your eyes.”

An eclipse shadow as seen from the ISSNASA

Still determined to shoot the eclipse? A wide-angle view will capture the surrounding landscape, making your photos unique (no one else will be standing exactly where you are). For that, you need a good tripod—and, ideally, a remote control so you can look at the eclipse while snapping pictures.

If you want a close-up of the eclipsed sun, you’ll need a telephoto lens (ideally with a focal length of at least 300 mm). Again, a solid tripod is essential. You’ll probably want to use a low ISO to keep the image sharp, and always “bracket”—that is, use exposures that are both shorter and longer than whatever your camera’s automatic setting tells you to use. (Sky and Telescope magazine has more tips for shooting the big event.)

Finally, because the scene will be changing so rapidly, video is another option (especially if you hit the record button and leave your camera to run on its own). Video records sound, of course, and when you play back the recording, you might be surprised just how excited you and your fellow eclipse-watchers got during those brief minutes of totality.

The next total solar eclipse will occur on July 2, 2019, but will be visible only from parts of Chile and Argentina. The next total solar eclipse visible in the U.S. will occur on April 18, 2024. It will be visible from a path running from central Texas to upstate New York.

But if you can't be there in person for this eclipse, you might want to watch it online. NASA TV will be showing the eclipse from start to finish. The agency is also planning a live-streamed, multi-platform “mega-cast” for those watching online.

It just might. Joe Rao, the meteorologist, says he’ll never forget his first total eclipse, which he saw from the shores of the St. Lawrence River in eastern Quebec in 1972. “It’s like how you always remember your first kiss,” he says. Zeiler adds: “It is Nature’s grandest spectacle. It’s unlike anything you’ve seen before. You will just be astounded.” And as Bakich puts it, a total eclipse “will stand out as one of the greatest—if not the greatest—sights you have or ever will behold.”